Cygwin: doc: Explictly name the output when building .info files

This works around a bug in docbook2x-texi seen with bash 5.3 while
cross-building from Fedora 42.

If the --output-dir option to db2x_texixml isn't specified (always the
case when invoked by docbook2x-texi), then the script attempts cd ''
which is now an error ("cd: null directory") rather than being treated
as equivalent to cd '.'

Instead, explicitly name the output file when building .info files.

Author: jon-turney

winsup/doc/Makefile.am

@@ -24,7 +24,7 @@ XMLTO_HTML_CHUNK_QUIET=$(XMLTO_HTML_CHUNK_QUIET_$(V))
 XMLTO_MAN_QUIET_=--stringparam man.output.quietly=1
 XMLTO_MAN_QUIET=$(XMLTO_MAN_QUIET_$(V))
 
-DOCBOOK2XTEXI=@DOCBOOK2XTEXI@ --xinclude --info --utf8trans-map=charmap
+DOCBOOK2XTEXI=@DOCBOOK2XTEXI@ --xinclude --info --utf8trans-map=charmap --to-stdout
 
 -include Makefile.dep
 
@@ -142,7 +142,7 @@ utils2man.stamp: $(cygwin-ug-net_SOURCES) man.xsl
 	@touch $@
 
 cygwin-ug-net.info: $(cygwin-ug-net_SOURCES) charmap
-	$(AM_V_GEN)$(DOCBOOK2XTEXI) $(srcdir)/cygwin-ug-net.xml --string-param output-file=cygwin-ug-net
+	$(AM_V_GEN)$(DOCBOOK2XTEXI) $(srcdir)/cygwin-ug-net.xml --string-param output-file=cygwin-ug-net >$@
 
 cygwin-api/cygwin-api.html: $(cygwin-api_SOURCES) html.xsl
 	$(AM_V_GEN)$(XMLTO) html -o cygwin-api/ -m $(srcdir)/html.xsl $(XMLTO_HTML_CHUNK_QUIET) $<
@@ -155,7 +155,7 @@ api2man.stamp: $(cygwin-api_SOURCES) man.xsl
 	@touch $@
 
 cygwin-api.info: $(cygwin-api_SOURCES) charmap
-	$(AM_V_GEN)$(DOCBOOK2XTEXI) $(srcdir)/cygwin-api.xml --string-param output-file=cygwin-api
+	$(AM_V_GEN)$(DOCBOOK2XTEXI) $(srcdir)/cygwin-api.xml --string-param output-file=cygwin-api >$@
 
 # this generates a custom charmap for docbook2x-texi which has a mapping for &reg;
 charmap: